This invention relates to flame retardant epoxy resin compositions, in particular curable flame retardant epoxy resin compositions comprising epoxy resin and an epoxy resin chain-extending amount of a diaryl alkylphosphonate and/or diaryl arylphosphonate and a cross-linking agent. The curable flame retardant epoxy resin compositions are particularly useful in e.g., coatings, adhesives, and composites.
Flame retardant epoxy resins are used in a variety of electrical insulating materials due to the excellent self-extinguishing properties, mechanical properties, water-vapor, resistance and electrical properties. It is conventional in the preparation of epoxy-containing laminates to incorporate into the epoxy resin composition various additives to improve the flame-retardancy of the resulting laminate. Many types of flame retardant additives have been suggested, however, the additives which are most widely used commercially are halogen-containing additives, such as tetrabromobisphenol A, or epoxy resins prepared with tetrabromobisphenol A.
Although halogen-containing fire-retardant additives such as tetrabromobisphenol A are effective, they are considered by some to be undesirable from an environmental standpoint, and in recent years there has been increasing interest in the formulation of halogen-free epoxy resins, which are able to meet the fire retardancy requirements which is typically V-0 in the standard “Underwriters Laboratory” test method UL 94.
There are some commercially available phosphorus-based fire retardant additives which may be useful for replacing halogen-containing fire-retardant additives. For example, by incorporating an addition-type phosphorus system flame-retardant such as triphenyl phosphate (TPP), tricresyl phosphate (TCP), cresyldiphenyl phosphate (CDP), resorcinol bis(diphenyl phosphate) (RDP), bisphenol A bis(diphenyl phosphate) (BDP) and the like which are a phosphate system compound, into an epoxy resin composition, the flame retardancy can be achieved. Examples of such formulations are described, for example, in U.S. Pat. Nos. 5,919,844; 5,932,637; 6,348,523; 6,713,163 and European Patent Application 1,359,174. However, since phosphorus compounds such as those described above do not react with an epoxy resin, other problems arise such as, decrease in solder heat resistance after moisture absorption and the reduced resistance to chemicals such as the alkali resistance of molded articles. Because of significant plasticizing effect of these phosphorus additives glass transition temperature (Tg) of the cured epoxy resin also finds significant drop.
Proposals have been made to use reactive phosphorus-based flame retardants instead of halogenated fire retardants in epoxy resin formulations. Overview of the state-of-the-art in phosphorus-based flame retardant epoxy resins was given in “Review on thermal decomposition, combustion and flame-retardancy of epoxy resins” by S. Levchik and E. Weil, Polymer International, Vol. 53, 2004, pp. 1901-1929. In some formulations phosphorus flame retardant was pre-reacted with an epoxy resin to form a di- or multifunctional epoxy resin which is then cured with a cross-linker.
The prior art describes the use of certain phosphorus-containing compounds as crosslinking or curing agents for use with epoxy resins as a way to introduce a phosphorus element into epoxy resin systems. For example, U.S. Pat. Nos. 4,973,631; 5,086,156; 6,403,220; 6,740,732; 6,486,242; 6,733,698 and 6,887,950 describe the use of difunctional or trifunctional phosphine oxides as effective curing agents. The above-mentioned prior art compositions are not easily prepared and require tedious preparation procedures.
The most often utilized phosphorus-based flame retardant for epoxy resins is 9,10-dihydro-9-oxa-10-phosphenanthrene 10-oxide (DOPO). There are two commonly known methods of applying DOPO to epoxy composites. In the first method DOPO is pre-reacted with epoxy resin as described in European Patent Application 0,806,429 and U.S. Pat. Nos. 6,645,631; 6,291,627 and 6,486,242. Because DOPO is a monofunctional reactive compound it terminates epoxy chains and therefore only multifunctional, usually more expensive then difunctional epoxies, must be used in this process. In the second method DOPO is pre-reacted with quinone or ketone type of compounds, having apart from these functionalities also two or more hydroxyl groups or amine groups as described, for example, in European Patent Applications 1,103,575 and 1,537,160 and U.S. Pat. Nos. 6,291,626; 6,441,067; 6,933, 050; 6,534,601; 6,646,064; 6,762,251 and 6,984,716 and in PCT Patent Publication 05/118604. This method suffers from the complexity which results in expensive compounds with low phosphorus content.
It would be therefore advantageous to provide a flame-retardant phosphorus-containing compound that could be derived from practical, industrial scale raw materials; and thus, would offer an economic advantage over the prior art processes.
Alkyl and aryl phosphonates in general are compatible with epoxy resins. In particular lower alkyl phosphonates are of value because they contain a high proportion of phosphorus, and are thus able to impart good fire retardant properties upon resins in which they are incorporated. Examples of use of the phosphonates in epoxy resins are shown for example in PCT Patent Publication 99/00451 and European Patent Application 0,758,654. However, if phosphonates are used as additives they suffer similar problems as non-reactive phosphates described above. The main problems with non-reactive phosphonates are low glass transition temperature and high moisture absorption of epoxy compounds. The laminates containing high levels of moisture tend to blister and fail, when introduced to a bath of liquid solder at temperatures around 260° C. for lead-based solder or around 288° C. for lead-free solder, a typical step in the manufacture of printed wiring boards.
Use of hydroxyl-terminated poly(m-phenylene methylphosphonate) in epoxy systems was described in PCT Patent Publication 03/029258. Here the epoxy resin was cured by poly(m-phenylene methylphosphonate) in the presence of a methylimidazole curing catalyst. PCT Patent Publication 04/060957 discloses the above-mentioned polyphosphonate which has been pre-reacted with epoxy resin, prior to curing process. Furthermore, this polyphosphonate effectively cures epoxy resin as described by T. Wu, A. M. Piotrowski, Q. Yao and S. V. Levchik in Journal of Applied Polymer Science, Vol. 101, pp. 4011-4022. Because the phosphonate is effectively incorporated in the epoxy network the final cured composite shows high glass transition temperature and low water absorption. PCT Patent Publication 04/060957 describes a process of pre-reaction of epoxy resin with poly(m-phenylene methylphosphonate), however because this polyphosphonate is a multifunctional compound it tends to cross-link epoxy resin and therefore pre-reaction cannot be effectively controlled on commercial scale. S. Minegishi et al describe the reaction of epoxy compounds with phosphonates in Journal of Polymer Science, Part A, Polymer Chemistry, Vol. 37, pp. 959-965. Their goal was to prepare high molecular weight polymeric linear phosphonates with little or no residual epoxy. Such polymers can be used as additive type flame retardants, but cannot be cross-linked because of low concentration or absence of epoxy groups.
Phosphorus-containing compounds have been used heretofore in a variety of epoxy-based polymer systems including, for example, FR-4 laminates for printed circuit boards, protective coatings, as well as structural and decorative composite materials. A driving force for this work has been the search for non-halogenated alternatives to brominated epoxy resins. However, recently another problem associated with electronic materials has been raised. That is, release of lead used in a solder material into the natural environment has become a serious problem and, as one strategy thereto, the use of a lead-free solder has been initiated. Pursuant to this, a solder treating temperature is elevated higher by about 10 to 15° C. than the previous temperature and, thus, there arises a difficulty that the aforementioned techniques cannot deal therewith because of low thermal stability of convenient brominated epoxy resins.
In light of the limitation of the prior art, an object of the present invention is to provide curable flame retardant epoxy resin compositions for use in, e.g., the manufacture of printed-wiring boards and multilayer printed-wiring boards, which does not produce harmful substances upon burning, provides good solder heat resistance after moisture absorption and superior adhesive abilities.